In rotary drilling of subterranean wells a drilling fluid, also referred to as drilling mud, is used for various functions, such as suspending and removing cuttings from the well, controlling formation pressures, sealing permeable formations, maintaining wellbore stability, minimizing formation damage, cooling, lubricating and supporting the bit and drilling assembly, transmitting hydraulic energy to tools and bit, ensuring adequate formation evaluation, controlling corrosion of the drilling assembly as far as possible, and facilitating cementing and completion of the well. A further, more recent, aspect is minimizing the effect on the environment.
Drilling fluids are well known from the prior art. They are typically classified according to their base material. In oil base fluids, solid particles are suspended in oil, and water or brine may be emulsified with the oil, usually assisted by a surfactant, whereby the oil is typically the continuous phase. In water base fluids, solid particles are suspended in water and oil may be emulsified therewith, usually in the presence of a surfactant. Oil based drilling fluids are generally used in the form of invert emulsion muds, where the oil phase typically forms the continuous phase. Earlier drilling fluids relied on heavier oils derived from petrochemicals, such as diesel oil. Such oils are now less favored because of their negative environmental impact and there have been a number of publications in the last few years proposing biodegradable drilling fluids or drilling fluid components.
Drilling fluids comprising reduced toxicity hydrocarbon as main component of the oil phase are known, for example from EP 449 257 A2, U.S. Pat. No. 5,569,642 and U.S. Pat. No. 4,787,990. One disadvantage of such systems is, however, that increased amounts of structure building agents such as viscosifiers are often required in order to obtain suitable rheological properties for a drilling fluid.
DE 39 07 391 A1, DE 39 07 392 A1, DE 38 42 703 A1 and DE 38 42 659 A1 describe inverse drilling fluids comprising ester as main component of the oil phase. These drilling fluids are distinguished by improved lubricity and biodegradability compared to mineral oil-based drilling fluids.